1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as switching devices.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and an elasticity, etc. Such characteristics can be explained by the following equations (1) and (2):τr∝γd2/Δε|Va2−VF2|where τr represents a rising time when a voltage is applied to a liquid crystal, Va is an applied voltage, VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of liquid crystal cells, and γ represents a rotational viscosity of the liquid crystal molecules.τf∝γd2/Kwhere τr represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an elastic constant.
A twisted nematic (TN) mode liquid crystal has a response time altered due to physical characteristics of the liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
Referring to FIG. 1, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in the data by using a look-up table (hereinafter referred to as high-speed driving strategy). This high-speed driving scheme allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. This high-speed driving scheme increases ** from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating a data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
In other words, the high-speed driving scheme compares most significant bits of the previous frame Fn-1 with those of the current frame Fn to select corresponding modulated data Mdata from the look-up table if there is a change in the most significant bits MSB, as shown in FIG. 3. This high-speed driving scheme modulates only several most significant bits so as to reduce a capacity burden of a memory upon implementation of hardware equipment. A high-speed driving apparatus in this manner is as shown in FIG. 4.
Referring to FIG. 4, a conventional high-speed driving apparatus includes a frame memory 43 connected to the most significant bit bus line 42 and a look-up table 44 commonly connected to the most significant bit bus line 32 and an output terminal of the frame memory 43.
The frame memory 43 stores most significant bit data MSB during one frame interval and supplies the stored data to the look-up table 44. Herein, the most significant bit data MSB may be the most significant 4 bits of the 8-bit source data RGB.
The look-up table 44 compares most significant bits MSB of a current frame Fn inputted from the most significant bit line 42 with those of the previous frame Fn-1 inputted from the frame memory 43 as shown in Table 1 or Table 2, and selects the corresponding modulated data Mdata. The modulated data Mdata are added to least significant bits LSB from a least significant bit bus line 41 to be applied to the LCD.
TABLE 10123456789101112131415 0023456791012131415151515 1013456781012131415151515 2002456781012131415151515 3001356781011131415151515 400124678 911121314151515 500123578 911121314151515 600123468 910121314151515 700123457 910111314151515 800123456 810111213151515 900123456 7 91112131415151000123456 7 81012131415151100123456 7 8 911121415151200123456 7 8 910121415151300123345 6 7 810111315151400123345 6 7 8 9111214151500112334 5 6 7 8 9111315
TABLE 1016324864809611212814416017619220822424000324864809611214416019220822424024024024016016486480961121281601922082242402402402403200326480961121281601922082242402402402404800164880961121281601762082242402402402406400164864961121281441761922082242402402408000163248801121281441761922082242402402409600163248649612814416019220822424024024011200163248648011214416017620822424024024012800163248648096128160176192224240240240144001632486480961121441761922082242402401600016324864809611212816019220822424024017600163248648096112128144176208224240240192001632486480961121281441601922242402402080016324848648096112128160176208240240224001632484864809611212814417619222424024000016324848648096112128144176208240
In the above tables, a furthermost left column is for a data voltage VDn-1 of the previous frame Fn-1 while an uppermost row is for a data voltage VDn of the current frame Fn. Table 1 is look-up table information in which the most significant bits (i.e., 20, 21, 22 and 23) are expressed by the decimal number format. Table 2 is look-up table information in which weighting values (i.e., 24 25, 26 and 27) of the most significant 4 bits are applied to 8-bit data.
However, the conventional high-speed driving scheme has a problem in that, since it looks for the modulated data Mdata registered in the look-up table using the look-up table comparing only the most significant bits, a continuity of the modulated data Mdata is more deteriorated due to a deviation from a real gray scale of the video data. In addition, a data overshoot may be caused between the adjacent modulated data Mdata. For this reason, values of the modulated data Mdata at gray level portions indicated by arrows in FIG. 5 are jumped between a gray level of the real input data and a gray level of the modulated data Mdata, thereby causing a larger brightness variation. In order to solve this problem, it is necessary to enlarge a memory size of the frame memory and the look-up table to compare full bits (i.e., 8 bits) of source data, so that full-bit modulated data selected can be derived in accordance with the compared result. However, such a full-bit comparison raises another problem of enlarging a memory size of the frame memory and the look-up table. As a result, a cost required for a circuit configuration increases in the full bit data modulation. For instance, a look-up table comparing 8-bit source data to select 8-bit modulated data Mdata has a memory size of 65536×8=524 kbits.